The present invention relates to thermodynamic conversion systems and, more particularly, to thermodynamic conversion systems employing large gas turbines. The invention is especially adapted to a cogeneration system in which a gas turbine produces an output torque and a large quantity of hot exhaust gasses. The energy in the hot exhaust gasses is captured in an associated heat recovery steam generator.
A cogeneration system typically uses the torque output of the gas turbine for generating electric power or for some other consuming process. Steam generated by the heat recovery steam generator may be applied to an auxiliary steam turbine for generating additional torque, or applied to a using process which is capable of direct use of the steam without intermediate conversion to torque. The total output of a cogeneration system offers an attractive thermodynamic conversion efficiency.
As is well known, improved efficiency in an internal combustion engine, including a gas turbine engine, is attained with lean fuel-air mixtures at elevated combustion temperatures. Unfortunately, elevated combustion temperatures increase the production of NOx compounds in the exhaust. The desire to improve efficiency thus comes into conflict with the desire to reduce NOx compounds in the exhaust. Government regulations increasingly place more stringent limits on the amount of NOx which an internal combustion engine is permitted to emit into the atmosphere.
NOx emissions can be reduced by brute-force methods such as, for example, reacting the exhaust with a catalyst or adding additional chemicals to the exhaust stream to react the NOx components into chemical compounds which are more easily removed from the exhaust stream before it exits the stack. Catalytic reaction and chemical addition both require additional structure and operating expense to support.
It is conventional to employ water or steam injection into the combustion zone of a gas turbine combustor to reduce the reaction temperature and thereby to reduce NOx emission.
It is also conventional to inject excess steam, which may not otherwise be needed in the using process of a cogeneration system, into the output of the compressor of the gas turbine engine. The mass flow added to the output of the compressor augments the power produced by the turbine portion of the gas turbine engine. A portion of the steam thus injected into the output of the compressor for power augmentation enters the combustion zone of the combustor. This steam also reduces the combustion reaction temperature and thereby the NOx emissions.